Level meters, particularly guided wave and capacitance meters, are well known in the art as devices for determining liquid levels in tanks. Generally, these devices consist of a probe contained in a tank, and electronics for generating/detecting signals. In the guided wave meter, a generated signal is coupled to the probe, and transmitted down the probe, using time domain reflectometry principles. When the signal reaches a fluid interface in the tank with a corresponding change in dielectric constants, a reflection is generated which travels up the probe to be detected by the instrument's electronics. The travel time is converted into a usable format representative of the level of fluid within the container or tank represented by the fluid interface. In capacitance level meters, the probe consists of two separated conductive members, and the meter measures the capacitance between the members. The capacitance changes between the members based upon the fluid fill and fluid levels between the conductive members. In either meter, the probe is placed in the tank containing the product media or in a side chamber that is fluidly connected to the tank. The probe is generally a rigid rod or cable orientated vertically in the tank.
In a guided wave (such as a guided wave radar level meter), the probe helps signal propagation and reduce signal losses from the traveling electromagnetic signal propagating down the probe as the probe affords a highly efficient path for pulse travel so that degradation of the signal is minimized. Further, because the pulse signals are channeled by the probe, turbulence or tank obstructions should not affect the measurement. Guided wave radar can handle varying specific gravity and media buildup or coatings. It is an invasive method, though, and the probe may be damaged by the blade of an agitator or the corrosiveness of the material being measured. Specially designed probe configurations allow extremely low dielectric materials (K<1.7 vs. K=80 for water) to be effectively measured.
One configuration used to measure low dielectric materials and to further reduce signal losses of the traveling pulse is to position the probe inside an outer metal jacket, such as a cylinder (or a stilling-well), creating a coaxial cable type structure having on outer cylindrical shield member, annular gap, and center positioned conductor. The conductor is electrically connected to the signal generation and reception electronic. Fluid enters the annular gap though openings in the outer shield member. Hence, the liquid level forms an interface within the coaxial structure to be detected by the pulse traveling on the surface of the conductor. Generally the outer shield member is an inert media sleeve, such as an aluminum or stainless steel tube that forms a concentric tube surrounding the entire enter conductor length. The outer shield functions as a further wave guide and acts as a ground plane to help channel the energy coupled to the conductor, thereby allowing the sensor to detect more subtle dielectric changes and correctly indicate the level of the product in the tank. Unfortunately, such a configuration can be subject to buildup or caking of product within the annular space which potentially can result in obstruction of the annular space and a non-functioning or malfunctioning device.
One design used to overcome the buildup of product or media in the annular space is to use a rod or cable running parallel with the conductor as the ground plane, instead of a concentric tube. The single parallel cable or rod helps to contain or channel the transmitted and reflected energy pulses near the vicinity of the probe. The parallel rod is more effective that a single rod or cable in air, but less effective than a device using the outer concentric shield as an additional waveguide. The dual parallel rod or cable probe is not as susceptible to media build up between the cables or rods as is the coaxial type structure.
Guided wave transmitters and capacitance meters are suitable for installation in deep tanks or silos, such as 100 feet or deeper. However, placement of the probe in large tanks requires suitable clearance. Hence, for tanks in excess of 30-50 feet, a rigid probe is problematic from a handling and installation standpoint. For this reason, in deep tanks or installations lacking sufficient clearance, flexible cable probe is generally used. However, in deep tanks, attenuation or energy loss in a guided wave device can be severe, and hence, a dual cable design waveguide construction, such as shown in FIG. 8, can help to reduce energy loss. However a more efficient flexible probe device is needed both for a guided wave level meters and a capacitance type level meter.